How to implement a java tester class

I implement an email system I have a main application class which is scanning some input from the keyboard and while input is not "q" ı have to able to add any command from console. The problem is how can i implement a tester class without changing the main application class. (I need to use already prepared commands within the tester class. They wont be entered explicitly, this means that when we test the code the prepared commands entered own its own). How can do this ? Or is it possible ?

I have 2 main method and I'm not allowed to use Junit.